Elastomers such as ethylene-propylene copolymer (EPM) and ethylene-propylene-diene (EPDM) are commercially produced by three different processes: the solution process, the suspension process (also known as the slurry process), and the gas-phase process using Ziegler-Natta catalyst systems based on transition metals.
In a solution process, the polymer formed is dissolved in the polymerization medium. The higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing polymer, monomers and solvent. High viscosity in the polymerization reactor associated with solution process is a limiting step for process efficiency and polymer production. High viscosity can also lead to the difficulty for efficient mixing in the reactor to maintain a homogeneous system and avoid product property drift (heterogeneity), reactor safety, process control problems. This is especially true to process with molecular weight polymers. Higher operating temperature reduces the viscosity. However, the molecular weight of the polymer tends to decrease with reaction temperature. Thus production of higher molecular weight polymers in solution processes is generally limited by the viscosity of the polymerization medium.
Polymers with low crystallinity or amorphous character as well as those with higher crystallinities are currently produced in slurry and suspension processes using hydrocarbon solvents as diluents. The suspension process can advantageously handle polymer concentrations in the reactor up to 25-30 weight %, as compared to 7-18 weight % in the solution process which are limited by solution viscosity at a high level of solids content. These higher concentrations are attributable to the characteristics of decreasing polymer-polymer entanglements in the suspension process by forcing the entangled polymer into the dispersed suspended phase thereby decreasing the concentration of entangled polymer in the bulk reactor solution phase. The recovery of polymer is also simpler in the slurry process. However, it is necessary that the solid particles in the slurry process do not agglomerate to one another or adhere to the surfaces of the reactor wall and the transport lines. Extremely low operating temperatures are adopted reduce the soluble fraction of polymer in the solvent. Mitigation of polymer fouling is a challenging task.
High monomer content polymer inherently has low melting or softening temperature. Under such conditions in either a fluidized or stirred gas-solid phase reactor, stickiness of the olefin polymer particles or granules becomes a problem. Ethylene copolymers using propylene, butene-1, and higher alpha comonomers are prone to stickiness problems when their crystallinity is below 30%. The stickiness problem becomes even more critical with copolymers of ethylene and propylene having a crystalline content less than 10%. In the gas-phase process, polymerization is carried out in the presence of a Ziegler-Natta catalyst, which can either be charged as a prepolymerized catalyst system or can be non-prepolymerized. In the latter case, using a fluidization aid, such as carbon black, talc or silica, is necessary to maintain stable fluidization and to avoid agglomeration of polymer particles and sheeting on the reactor walls. Using carbon black limits the production to black elastomer product.
Many polymers are insoluble in the reaction medium from which they are formed. Upon significant polymerization, polymer chains reach a crystallizable length and polymer nucleation and crystallization begins. The crystallization of polymers leads to polymer-solvent phase separation. On the other hand, polymer-solvent phase separation can be also induced through change of solvency of the reaction medium with respect to polymer. Swell and stickiness of the separated polymers can be improved through change of the affinity between the polymer and reaction medium. The instant invention provides a polymerization process which efficiently produces polymer with reduced operation difficulties and/or reduced reactor fouling. This is achieved through proper selection of diluent in reaction medium.
Processes initially used for the manufacture of polypropylene were based on the use of a hydrocarbon diluent to suspend crystalline polymer particles formed in the process and dissolve the amorphous polymer fraction. Residual catalyst components were deactivated and solubilized by treatment with alcohol, and the deactivated catalyst separated from the diluent by treatment with water. The crystalline polymer product was separated from the diluent by filtration or centrifugation and then dried. The amorphous polymer, which was soluble in the diluent, was separated by evaporation.
What has been referred to as fourth generation polymerization catalysts in Polypropylene Handbook, Edward P. Moore, Jr., Ed., Hanser Publishers, 1996, have led to processes that do not require the use of diluents in the polymerization process by using either liquid or gaseous monomer as the polymerization medium. However, flexibility of types of catalysts used in non-diluent systems is somewhat limited and higher quality propylene feed is generally required.
The stickiness of polymer can be mitigated through reducing the granule swell and improved particle morphology. An example of a polymerization process that incorporates the use of a nonreactive diluent is shown in U.S. Pat. No. 3,470,143 (Schrage et al.). Specifically, the Schrage patent discloses the use of a fluorinated organic carbon compound as a diluent in polymerizing at least one ethylenically unsaturated hydrocarbon monomer to form an amorphous elastomer. The product can be dried in the form of small particles.
EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature.
U.S. Pat. No. 3,056,771 discloses polymerization of ethylene usingTiCl4/(Et)3Al in a mixture of heptane and perfluoromethylcyclohexane, presumably at room temperature.
U.S. Pat. No. 5,624,878 discloses the polymerization using “constrained geometry metal complexes” of titanium and zirconium.
Adhesion of polymers to reactor walls in slurry polymerization processes is and has been a known problem. Japanese Kokai Patent Application SHO 61[1986]-7301 indicates that a prior method of reducing this adhesion or fouling problem was to keep the slurry concentration at a relatively low level. However, such a process would have to be run at a relatively low polymer productivity. A further method of reducing polymer adhesion that is also described in the '7301 Kokai is to use a certain concentration of fluorocarbons in the polymerization system. The amount of fluorocarbon used in the system is generally limited from 0.01-5weight %. Below this amount, the use of the fluorocarbon is said to be ineffective and above this amount is reported to result in lower polymerization activity.
There remains a need to increase polymer product quality and process efficiency, particularly processes that reduce slurry polymerization fouling without suffering any substantial loss in polymerization activity. It is particularly desirable to find polymerization processes that use propylene as at least one monomer feed component, and to produce a polypropylene copolymer type product that can be recovered in particle form. Such a process would also be desirable in the production of propylene-ethylene type polymers that have significant amounts of ethylene incorporated into the copolymer. Processes that provide for higher flexibility in types of catalyst that can be used, as well as provide copolymers that are very low in crystallinity, whether at low or high amounts of ethylene incorporation, are especially preferred. In addition, processes that provide for the use of diluents are also preferred in that such processes would generally provide an increase in comonomer (e.g., ethylene) content at a lower comonomer partial pressure.